The present invention relates to a grease for sealing a ball-and-roller bearing built in a wide range of industrial equipment and automobiles, and a grease sealed bearing in which the bearing is sealed with this grease. More particularly, the present invention relates to a grease and a grease sealed bearing suitable for a ball-and-roller bearing used at high temperature and high speed rotation.
The ball-and-roller bearing built in the industrial equipment and the automobiles is sealed with a grease for lubricity. The grease is obtained by mixing a base oil and a thickener. Typical base oil is a synthetic lubricating oil such as a mineral oil, an ester oil, a silicone oil, and an ether oil. Typical thickener is a metal soap such as lithium soap, and an urea compound. Various additives such as an antioxidant, a rust preventive, a metal deactivator, and a viscosity index improver are added to the grease, as required.
In recent years, a demand for light-weighed automobiles urges to miniaturize and lighten electrical equipment of the automobiles. On the other hand, the electrical equipment of the automobile is required to provide high output and high efficiency. In the electrical equipment of the automobiles, a decrease in output accompanied by the miniaturization is compensated with a high speed rotation.
Accordingly, the ball-and-roller bearing is required to withstand the high speed rotation and a high load. The lifetime of the grease for sealing the bearing is generally shorter than that of the bearing itself. In other words, the lifetime of the bearing itself depends on that of the grease. There is a need to provide the grease having prolonged lifetime for preventing seizure at high temperature under the high speed and the high load.
Japanese Patent Laid-Open Application No. 60-31598 discloses a grease for sealing a ball joint for lowering operation torque at high temperature, comprising (A) 100 parts by weight of poly-α-olefin-based synthetic oil having viscosity of 500 to 2000 sct at 40° C., (B) 6 to 30 parts by weight of at least one wax selected from a group consisting of paraffin wax, polyethylene wax, microcrystalline wax and fatty acid amide wax, and (C) 1 to 50 parts by weight of an urea-based thickener. In fact, the boll joint grease uses mixed wax comprising two or four types of the above-mentioned waxes. Such ball joint grease has insufficient lifetime at high temperature under severe use conditions, especially under the high speed and the high load, accompanied by with the recent miniaturization and high performance of various machine members. An antioxidant or an extreme pressure agent is conventionally added to the grease in order to improve the lifetime at high temperature. However, sufficient effects are not yet achieved.